U.S. patent application number 14/888879 was filed with the patent office on 2016-03-24 for nano-electrode multi-well high-gain avalanche rushing photoconductor.
The applicant listed for this patent is THE RESEARCH FOUNDATION FOR THE STATE UNIVERSITY OF NEW YORK. Invention is credited to Amirhossein GOLDAN, John A. ROWLANDS, Wei ZHAO.
Application Number | 20160087113 14/888879 |
Document ID | / |
Family ID | 51989392 |
Filed Date | 2016-03-24 |
United States Patent
Application |
20160087113 |
Kind Code |
A1 |
GOLDAN; Amirhossein ; et
al. |
March 24, 2016 |
NANO-ELECTRODE MULTI-WELL HIGH-GAIN AVALANCHE RUSHING
PHOTOCONDUCTOR
Abstract
Provided is a detector that includes a scintillator, a common
electrode, a pixel electrode, and a plurality of insulating layers,
with a plurality of nano-pillars formed in the plurality of
insulating layers, a nano-scale well structure between adjacent
nano-pillars, with a-Se separating the adjacent nano-pillars, and a
method for operation thereof.
Inventors: |
GOLDAN; Amirhossein; (Middle
Island, NY) ; ZHAO; Wei; (East Setauket, NY) ;
ROWLANDS; John A.; (Ontario, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
THE RESEARCH FOUNDATION FOR THE STATE UNIVERSITY OF NEW
YORK |
Albany |
NY |
US |
|
|
Family ID: |
51989392 |
Appl. No.: |
14/888879 |
Filed: |
May 29, 2014 |
PCT Filed: |
May 29, 2014 |
PCT NO: |
PCT/US14/39992 |
371 Date: |
November 3, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61828350 |
May 29, 2013 |
|
|
|
Current U.S.
Class: |
250/370.12 ;
257/42; 438/84 |
Current CPC
Class: |
G01T 1/24 20130101; H01L
27/14676 20130101; H01L 31/18 20130101; H01L 31/0376 20130101; H01L
31/117 20130101; H01L 31/0272 20130101 |
International
Class: |
H01L 31/0272 20060101
H01L031/0272; H01L 31/18 20060101 H01L031/18; G01T 1/24 20060101
G01T001/24; H01L 31/0376 20060101 H01L031/0376; H01L 31/117
20060101 H01L031/117 |
Claims
1. A detector comprising: a common electrode; a pixel electrode;
and a plurality of insulating layers, wherein a plurality of
nano-pillars are formed in the plurality of insulating layers, with
a nanoscale well structure between adjacent nano-pillars, and
wherein amorphous selenium (a-Se) separates the adjacent
nano-pillars.
2. The detector of claim 1, wherein the a-Se fills the nanoscale
well structure.
3. The detector of claim 1, wherein an avalanche transport region
separates the nano-pillars and the common electrode.
4. The detector of claim 1, further comprising an electron blocking
layer separating the nano-pillars and the nanoscale well structure
from the a-Se.
5. The detector of claim 4, wherein the electron blocking layer
defines a detection region.
6. The detector of claim 1, further comprising a scintillator
adjacent to the common electrode, on a side of the common electrode
opposite the plurality of insulating layers
7. The detector of claim 6, wherein avalanche gain occurs above the
electron blocking layer, outside of the detection region.
8. The detector of claim 1, further comprising a nano-electric
Frisch grid embedded within each nano-pillar.
9. The detector of claim 1, further comprising a hole blocking
layer between the common electrode and the a-Se.
10. A detection method comprising: detecting movement of holes in a
detection region of a detector that includes a scintillator, a
common electrode, a pixel electrode, a plurality of insulating
layers and a substrate, wherein the plurality of nano-pillars are
formed in the plurality of insulating layers, with a nanoscale well
structure between adjacent nano-pillars, and amorphous selenium
(a-Se) separates the adjacent nano-pillars.
11. The method of claim 10, wherein the a-Se fills the nanoscale
well structure.
12. The method of claim 10, wherein an avalanche transport region
separates the nano-pillars and the common electrode.
13. The method of claim 10, wherein an electron blocking layer
separates the nano-pillars and the nanoscale well structure from
the a-Se.
14. The method of claim 13, wherein the electron blocking layer
defines the detection region.
15. The method of claim 14, wherein avalanche gain occurs above the
electron blocking layer, outside of the detection region.
16. The method of claim 10, wherein a nano-electric grid is
embedded within each nano-pillar.
17. The method of claim 16, wherein the nano-electric grid is a
Frisch grid.
18. The method of claim 16, wherein the nano-electric grid masks
detection of electrons within the detection region.
19. The method of claim 10, wherein a hole blocking layer is
provided between the common electrode and the a-Se.
20. A method of manufacture of a detector that comprises a common
electrode, a pixel electrode, and a plurality of insulating layers,
the method comprising: forming a plurality of nano-pillars in the
plurality of insulating layers, with a nanoscale well structure
between adjacent nano-pillars, and injecting amorphous selenium
(a-Se) between the adjacent nano-pillars.
Description
PRIORITY
[0001] This application claims priority to U.S. Provisional Patent
Application No. 61/828,350 filed with the U.S. Patent and Trademark
Office on May 29, 2013, the content of which is incorporated herein
by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to solid-state
imaging detectors of ionizing radiation and, in particular, to
amorphous selenium radiation detectors having an ultra-fast photo
response and ultra-high time resolution.
[0004] 2. Description of the Related Art
[0005] Amorphous selenium (a-Se) was previously developed for
photocopying machines. A-Se has been commercially revived as a
direct x-ray photoconductor for Flat-Panel Detectors (FPDs) due to
high x-ray sensitivity and uniform evaporation over a large area as
a thick film. However, current direct conversion FPDs are limited
by, inter alia, degradation of low-dose imaging performance due to
electronic noise, because energy required to generate an
electron-hole pair in a-Se is 50 eV at 10 V/micron. Although other
photoconductive materials with higher conversion have been
investigated, the other photoconductive materials suffer from
charge trapping and manufacturing issues. Improved conversion of
a-Se is possible by increasing the electric field above 30
V/micron, i.e., 30,000 V on a 1,000 micron layer. However, such
electric field increase is extremely challenging for reliable
detector construction and operation, and is virtually
impractical.
[0006] High resistivity amorphous solids used as photoconductors,
especially amorphous selenium, are of interest because the high
resistivity amorphous solids are readily produced over a large area
at substantially lower cost than grown crystalline solids.
[0007] However, amorphous solids, i.e., non-crystalline solids with
disorder, have been ruled out as viable radiation imaging detectors
in a photon-counting mode because of low temporal resolution due to
low carrier mobilities and transit-time limited pulse response, and
low conversion gain of high energy radiation to electric charge.
Avalanche multiplication in selenium can be used to increase the
electric charge gain. However, significant obstacles prevent
practical implementation of a direct conversion a-Se layer with
separate absorption and avalanche regions.
[0008] A-Se has approximately 90% detection efficiency in blue
wavelength, making A-Se ideal for coupling to Time of Flight (TOF)
specific scintillators for high-energy radiation detection. A
drawback of a-Se is poor time-resolution and low mobility due to
shallow traps, problems that conventional devices have not
circumvented for TOF detectors.
[0009] Direct conversion x-ray Flat-Panel Imagers (FPIs) provide
high resolution and high detection efficiency, and detectors based
on active matrix Thin Film Transistor (TFT) array readout of
amorphous selenium photoconductor have been commercialized for
general radiographic as well as mammographic clinical applications.
However, conventional systems have only shown continuous and stable
avalanche multiplication in a-Se, a feature that enabled
development of an optical camera one hundred times more sensitive
than a state of the art Charge Coupled Device (CCD) camera. See, M.
M. Wronski, et al., Med. Phys. 37, 4982 (2010); and K. Tanioka, J.
Mater. Sci., Mater. Electron. 18, pp. 321-325 (2007).
[0010] Positron Emission Tomography (PET) is a nuclear medical
imaging modality that produces three dimensional (3D) images to see
functional processes in human body. PET is commonly used in
clinical oncology for detecting cancer, and for clinical diagnosis
of heart problems and brain disorders. After positron-emitting
radionuclides are introduced into the body, the radionuclides decay
with each annihilation emitting two photons in diametrically
opposing directions. TOF PET systems detect these photons, use TOF
information to determine if two registered photons are in time
coincidence, in which case the registered photons belong to a same
positron annihilation event, and use the arrival time difference to
localize each annihilation event. Without TOF localization data,
computationally expensive iterative reconstruction algorithms are
used to estimate 3D distribution of events that provide the best
match with the measured projection data. Localization accuracy
.DELTA.x of a TOF PET is determined by time-resolution At of the
radiation detector, according to .DELTA.x=c.DELTA.t/2, where c is
the speed of light.
[0011] An ultimate TOF detector, i.e., a TOF detector having a At
less than 10 picosecond (ps), has not been realized. Existing
commercial systems utilize PhotoMultiplier Tubes (PMTs) based on a
plano-concave photocathode, which only achieve a .DELTA.t of
approximately 500 ps. Silicon PhotoMultipliers (SiPMs), which are
based on Geiger mode operating avalanche photodiodes, have achieved
a better .DELTA.t, i.e., SiPM .DELTA.t.about.100 ps. However,
conventional systems suffer from high cost of PMTs and other
components, complicated plano-concave photocathode arrangements,
poor photon detection efficiency, optical crosstalk, small area,
and poor uniformity.
SUMMARY OF THE INVENTION
[0012] Accordingly, aspects of the present invention address the
above problems and disadvantages, and provide the advantages
described herein. An aspect of the present invention provides a TOF
detector that uses a-Se as the photoconductive material to provide
a radiation detector and method for operation of same that
overcomes disadvantages of conventional detectors. Another aspect
of the present invention provides a Nano-Electrode multi-Well
High-gain Avalanche Rushing Photoconductor (NEW-HARP).
[0013] An aspect of the present invention provides a detector that
includes a scintillator, a common electrode, a pixel electrode, and
a plurality of insulating layers, with a plurality of nano-pillars
formed in the plurality of insulating layers, a nano-scale well
structure between adjacent nano-pillars, with a-Se separating the
adjacent nano-pillars, and a method for operation thereof.
[0014] Another aspect of the present invention provides a detection
method that includes detecting movement of holes in a detection
region of a detector that includes a scintillator, a common
electrode, a pixel electrode, a plurality of insulating layers and
a substrate, with the plurality of nano-pillars being formed in the
plurality of insulating layers, with a nanoscale well structure
between adjacent nano-pillars, amorphous selenium (a-Se) separating
the adjacent nano-pillars, and the a-Se filling the nanoscale well
structure, to provide an avalanche transport region separating the
nano-pillars and the common electrode.
[0015] A further aspect of the present invention provides a method
for manufacture of a detector that includes a common electrode, a
pixel electrode, and a plurality of insulating layers, by forming a
plurality of nano-pillars in the plurality of insulating layers,
with a nanoscale well structure between adjacent nano-pillars, and
injecting amorphous selenium (a-Se) between the adjacent
nano-pillars.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The above and other aspects, features and advantages of
certain embodiments of the present invention will be more apparent
from the following detailed description taken in conjunction with
the accompanying drawings, in which:
[0017] FIG. 1 is a schematic side view of a picosecond detector
structure, according to an aspect of the present invention;
[0018] FIGS. 2(a)-2(m) illustrate fabrication of the photodetector,
according to an aspect of the present invention;
[0019] FIG. 3 is a chart showing weighting potential distribution
of the photodetector of the present invention and a conventional
Bipolar Planar Detector (BPD);
[0020] FIG. 4 is a chart showing induced photocurrent due to
impulse excitation of the photodetector of the present invention
and a conventional BPD;
[0021] FIG. 5 is a chart showing normalized integrated charge over
time of the photodetector of the present invention and a
conventional BPD; and
[0022] FIG. 6 is a schematic view of the photodetector of the
present invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0023] The following detailed description of certain embodiments of
the present invention will be made with reference to the
accompanying drawings. In describing the invention, explanation
about related functions or constructions known in the art are
omitted for the sake of clearness in understanding the concept of
the invention, to avoid obscuring the invention with unnecessary
detail.
[0024] Provided is a nano-pattern multi-well selenium detector that
combines avalanche multiplication and Unipolar Time-Differential
(UTD) charge sensing benefits in one device, and method for
manufacture of and detection utilizing the nano-pattern multi-well
selenium detector. The nano-pattern multi-well selenium detector,
i.e., photodetector, combines avalanche multiplication and UTD
charge sensing benefits in a single device fabricated by preparing
a nano-pattern substrate, growing a structure on the substrate, and
evaporating a-Se over the structure to form a photoconductive
avalanche layer, thereby providing a detector structure as a
nano-electrode multi-well high-gain avalanche rushing photodetector
that provides picosecond time resolution. The photodetector
structure includes high granularity micro-pattern multi-wells to
provide an improved fabricated UTD solid-state detector. As
described herein, time-of-light test results show, based on UTD
charge sensing, substantially improved detector time resolution and
achievement a At of less than 10 ps physical limit in signal rise
time, which is set by spreading of the photo-induced carrier
packet.
[0025] Aspects of the present invention provide avalanche
multiplication gain in amorphous semiconductors, due to impact
ionization in the presence of a strong electric field, i.e.,
exceeding 80 V/.mu.m, with the transport mechanism shifted from
localized states into extended states.
[0026] FIG. 1 provides a schematic side view of a picosecond
detector structure 100, according to an aspect of the present
invention. In FIG. 1, avalanche a-Se forms a photoconductive film
that fills nano-pattern multi-wells 150 for UTD charge sensing.
[0027] As shown in FIG. 1, a plurality of insulating nano-pillars
150 are preferably evenly spaced over pixel electrode 210, i.e.,
collector, showing an embodiment having first and second
nano-electrode Frisch grids 115, 116 embedded within respective
nano-pillars 150, formed by nano-patterning using electron beam
lithography, as described in regards to FIGS. 2(a)-2(m). Once the
nano-pattern substrate is prepared, a-Se is evaporated over the
substrate to provide photoconductive avalanche layer, forming a
nano-electrode multi-well high-gain avalanche rushing photodetector
structure. Also shown in FIG. 1 is an ultra-fast scintillator 295,
preferably formed of one of sodium iodide activated with thallium,
NaI(Tl), Bismuth Germanium Oxide (BGO), Gadolinium Silicate (GSO),
Lutetium Oxyorthosilicate (LSO), Lanthanum(III) bromide (LaBr3),
and Cerium- doped lanthanum chloride (LaCl3:Ce). The ultra-fast
scintillator 295 converts x-rays and gamma ray photons to optical
photons, which impinge on common electrode 110, which is discussed
below in regards to FIG. 2(m).
[0028] FIGS. 2(a)-2(m) are side profile views illustrating steps
for photodetector sensor structure fabrication. FIG. 2(a) shows a
pixel electrode 210 deposited via, e.g., Physical Vapor Deposition
(PVD), on substrate 200, which is preferably formed of glass or
silicon, patterned using optical lithography, to form a patterned
grid electrode. FIG. 2(b) shows a first insulator layer 220
deposited over pixel electrode 210 by, e.g., thermal growth, via
plasma-enhanced chemical vapor deposition, PVD, or spin
casting.
[0029] FIG. 2(c) shows deposition via, e.g., PVD, of first
electrode 230 over first insulator layer 220. FIG. 2(d) shows
removal, via nano-patterning and nano-lithography, e.g., Electron
Beam Lithography (EBL), of a center porition of first electrode 230
to create first grid patterned electrodes 232, 234. FIG. 2(e) shows
deposition of a second insulator layer 222 over first patterned
grid electrodes 232, 234, with first insulator layer 220 and second
insulator layer 222 forming a plurality of stacked insulating
layers.
[0030] FIG. 2(f) shows deposition of a second electrode 240 over
the second insulator layer 222, and FIG. 2(g) shows well-mask
electrode 242, 244 created by nano-patterning of second electrode
240. FIG. 2(h) shows first insulator 220 etched via, e.g., Reactive
Ion Etching (RIE), to create a nanoscale well structure 250.
[0031] As shown in FIG. 2(i), well-mask electrode 242, 244 is also
removed by etching, e.g., by RIE or chemical etching, and as shown
in FIG. 2(j), an electron blocking layer 260 is created over, and
conforms to inner contour of, each nano-scale well structure 250,
as shown in FIG. 6.
[0032] FIG. 2(k) and FIG. 6 illustrate a-Se evaporated over each of
the plurality of nano-scale well structures 250. FIG. 2(1) shows a
hole blocking layer 280 created over the a-Se avalanche
photoconductive layer 270, and FIG. 2(m) shows common electrode 110
that is created over hole blocking layer 280, which keeps holes
from entering into an avalanche transport region from the common
electrode 110, and is preferably formed of one of Bathocuproine
(BCP), Zinc Oxide (ZnO), Magnesium Oxide (MgO), and Cerium Oxide
(CeO). The electron blocking layer 260 is preferably formed of one
of Arsenic Triselenide (As.sub.2Se.sub.3) and Poly N-vinylcarbazole
(PVK).
[0033] Common electrode 110 is transparent to electromagnetic
radiation, e.g., optical photons, x-rays, and gamma rays. FIGS.
2(a)-2(m) show a single first patterned grid electrode 232, 234,
but more than one grid electrodes can be provided with additional
electrodes, as shown in FIGS. 1 and 6. Patterned grid electrodes
232, 234 form a nano-electrode grid that erases, i.e. masks,
electron detection within a detection region 290, which is
described below, thereby allowing for improved detection outside of
the avalanche transport region, which is a region of electron and
hole growth.
[0034] FIG. 3 is a chart of weighting potential distribution
comparing the detector of the present invention with a conventional
BPD. As shown in FIG. 3, weighting potential distribution shows
nearly ideal UTD charge sensing for the detector of the present
invention, with the conventional BPD shown for comparison. For
non-avalanche a-Se detectors with shallow-trap-limited drift
mobility, a charge broadening exists due to diffusion, mutual
Coulombic repulsion, and most importantly, fluctuations of
shallow-trap release time. See, A. H. Goldan, et al., Appl. Phys.
Lett. 101, p. 213503 (2012). However, for avalanche a-Se with
associated exponential impact ionization gain, as shown in the top
inset of FIG. 3, holes experience band transport in extended states
with non-activated microscopic mobility of 1 cm.sup.2Vs. Given that
bulk thickness (L) is very thin and operating field (F) is very
high for avalanche detectors, i.e., L=30 nm and F=100 V/fim, holes
experience negligible interruption by capture and thermal-release
events due to shallow traps. Furthermore, in the small-signal case,
spreading due to mutual Coulomb repulsion of the free charge
density can be neglected. The bottom inset of FIG. 3 shows finite
width of the V.sub.W distribution, close to pixel electrode 210 for
the nanopattern multi-well substrate 100, with impulse response
considering the limitations of charge spreading due to drift
diffusion and finite width of V.sub.W distribution.
[0035] FIG. 4 is a chart of induced photocurrent due to impulse
excitation comparing the detector of the present invention with a
conventional BPD, showing induced photocurrent due to impulse
excitation. The impulse response photocurrent of the conventional
BPD has a fast exponential component due to the drift of
high-mobility avalanched holes (130-136, FIG. 6) and a slow
rectangular component due to the drift of slow electrons. A
majority of avalanche gain occurs near the collector, where holes
are almost instantly neutralized, and a detected integrated charge
signal is obtained mainly from electron motion.
[0036] FIG. 5 is a chart of normalized integrated charge over time
comparing the detector of the present invention with a conventional
BPD. As illustrated in FIG. 5, the integrated current waveform,
i.e., collected charge, in BPD is temporally limited by electron
motion and signal rise time of approximately 50 ns. However, in the
photodetector of the present invention, which is labelled NEW-HARP
in FIGS. 3-5, the electrostatically shielding nano-pillars enable
UTD charge sensing. The photodetector yields impulse response time
of 20 ps at Full Width at Half Maximum (FWHM), as shown in FIG. 4,
and signal rise time of 25 ps, i.e., the time required for the
collected charge to rise from 10% to 90% of final value, as shown
in FIG. 5. Advanced nano-lithography allows for diffusion-limited
response with greater than three orders-of-magnitude improvement in
time resolution.
[0037] For a-Se operating in the avalanche mode at high electric
fields, charge drift occurs via band transport in the extended
states with non-activated microscopic mobility, and thus,
photocarriers experience negligible interruption by capture and
thermal-release events due to shallow traps. The implication of (1)
non-activated microscopic band-mobility, (2) avalanche gain, and
(3) UTD charge sensing show photo-detector achievement of 10 ps
time-resolution utilizing low-cost material that is uniformly
scalable to a large area.
[0038] FIG. 6 is a schematic view of the photodetector of the
present invention, also illustrating electron movement therein.
FIG. 6 shows a plurality of nanoscale well structures 250 and
provides an exploded view of multiplication gain in the avalanche
transport region formed by avalanche photoconductive layer 270 on
an side of a detection region 290 opposite substrate 200. As shown
in FIG. 6, the avalanche photoconductive layer 270 is formed
between detection region 290 and hole blocking layer 280, which
blocks injection of holes from common electrode 110 into the
photoconductive layer, e.g., a-Se, thereby reducing leakage
current.
[0039] Detection region 290 is provided within respective nanoscale
well structures 250 and, as shown in FIG. 6, as holes 130, 132,
134, 136 drift through the a-Se avalanche photoconductive layer
270, which forms an avalanche transport region, i.e., an avalanche
gain region in which the number of holes grow due to impact
ionization. Once in detection region 290, holes 138, 139 do not
substantially grow in size. Also shown in FIG. 6 is a decreasing
number of electrons 140, 142, 143 with increasing distance from the
detection region 290, with electron 143 being of maximum size
closest to detection region 290 and electron 140 being smallest in
size furthest from detection region 290.
[0040] Accordingly, a detector is provided that includes
scintillator 295, common electrode 110, pixel electrode 210, a
plurality of insulating layers, 220, 222, an electron blocking
layer 260 separating a plurality of nano-pillars 150 and the
nanoscale well structure 250 from the a-Se, with the electron
blocking layer defining detection region 290. The plurality of
nano-pillars 150 are formed in the plurality of insulating layers
220, 222, with nanoscale well structure 250 being formed between
adjacent nano-pillars and a-Se separating the adjacent nano-pillars
and filling the nanoscale well structure. Avalanche gain occurs
above the electron blocking layer, outside of the detection region,
with nano-electric Frisch grid 115, 116 embedded within each
nano-pillar, with the nano-electric grid masking detection of
electrons within the detection region.
[0041] Also, hole blocking layer 280 is provided between common
electrode 110 and the a-Se, with hole blocking layer 280 being
transparent to a wavelength of incoming photons, and possessing
virtually no hole transport to trap holes that are injected while
allowing electrons to exit a-Se to reach common electrode 110. Hole
blocking layer 280 can be provided via a plurality of layers
combining separate functions or by a single layer n-type polymeric
material having a wide gap. Geometry of the nano-grid and pillars
determine the electric field in the avalanche transport, i.e.,
gain, region 270 and in detection region 290, with field strength
and gain being variable by design geometry, a-Se thickness, common
electrode bias voltage, and nano-grid bias voltage, thereby
providing flexibility for broad application of the detector.
[0042] Accordingly, the present invention provides advantages of
avalanche mode a-Se having photo-conductive gain and band transport
in the extended states with the highest possible mobility and
negligible trapping. Importantly, UTD charge sensing enables
operating the detector at its theoretical limit of charge
diffusion, and provides UTD charge sensing with avalanche mode a-Se
improving At by more than three orders of magnitude for achievement
of 10 ps time-resolution with a material that is low-cost and
uniformly scalable to large-area.
[0043] An aspect of the present invention provides advantages over
conventional direct conversion FPIs that include providing
additional gain through impact avalanche, thus allowing the
detection of a single x-ray photon, and providing improved temporal
performance through unipolar sensing, thereby allowing quantum
noise limited performance for conventional x-ray integration
detection to be performed at a single x-ray photon level, and also
enables photon counting with excellent energy resolution and high
count rate, thereby allowing spectral imaging detectors made at
reduced cost compared to single crystal Cadmium telluride (CdTe)
detectors. Aspects of the present invention provide advantages over
conventional detectors provide applications in medical imaging,
e.g., TOF PET and particle physics, e.g., Cherenkov imaging
defectors and trackers, as well as optical communication and
time-domain spectroscopy.
[0044] While the invention has been shown and described with
reference to certain aspects thereof, it will be understood by
those skilled in the art that various changes in form and details
may be made therein without departing from the spirit and scope of
the present invention as defined by the appended claims and
equivalents thereof.
* * * * *